Dialkyl esters of 2,5-furandicarboxylic acid (2,5-FDCA) have found use as monomers in the preparation of bio-based polyesters. The preparation of such polyesters has been disclosed in e.g. US 2011/0282020 and WO 2013/062408. The preparation comprises the polycondensation reaction of an alkylene diol with the dialkyl ester of 2,5-FDCA. Alternatively, the acid can be used as monomer. When the acid is used as monomer it still can be convenient to prepare the dialkyl ester first and subject the dialkyl ester obtained to saponification, since the purification of crude 2,5-FDCA appears to be more cumbersome than the purification of the esters. The resulting polyester can be used as alternative for polyethylene terephthalate (PET), as stated in WO 2013/062408.
The preparation of the dialkyl ester of 2,5-FDCA is known in the art. In this context reference is made to e.g. EP2486027, describing a process wherein from a carbohydrate an ether of 5-hydroxymethylfurfural is prepared, which ether is subsequently oxidized to a product containing 2,5-FDCA and esters thereof. The product may be further esterified to yield an ester composition comprising the dialkyl ester of 2,5-FDCA and the monoalkyl ester thereof. It was found that the ester composition also typically contains some contaminants that cause coloration of the resulting polymer.
In order to enable the manufacture of transparent, colorless polyesters it is therefore desirable to purify the ester composition. It is known that the purification of the corresponding monomers for the preparation of PET can be accomplished by the distillation or crystallization of a crude ester composition of dimethyl terephthalate. In U.S. Pat. No. 4,040,793 it is disclosed that crystallization and recrystallization involve considerable cost since each recrystallization step requires four steps: the creation of a genuine solution, the crystallization as such, then the separation of the crystallized material and finally the washing of the crystals obtained. In U.S. Pat. Nos. 4,683,034 and 5,542,372 processes for the purification of dimethyl terephthalate are described wherein dimethyl terephthalate is separated from the dimethyl esters of isophthalic acid and orthophthalic acid by means of a combination of evaporation, crystallization and melt crystallization. These patent documents are silent about coloration-causing compounds.
Also as to the process according to EP 1380566 it was proposed that for the purification of dimethyl terephthalate melt crystallization is used. However, it appeared that melt crystallization alone is insufficient to obtain a sufficiently pure product. EP 1380566 therefore discloses a process that includes the esterification of the oxidation product of p-xylene and methyl p-toluate to yield a crude ester product, a distillation step to separate a light fraction from the crude ester product, a flash distillation to separate heavy, coloration-causing compounds from the crude ester product and at least two melt crystallization steps to obtain purified dimethyl terephthalate from the crude ester product. Between the two melt crystallization steps, the crystallization product of the first melt crystallization is again treated in an additional distillation column to remove coloration-causing compounds therefrom before being subjected to the second melt crystallization. This process shows the difficulty of the removal of coloration-causing compounds.